Video Transcript
Detergent molecules gather at the surface of the water when they are first added to it. Which illustration best describes the way in which they initially accumulate in the water?
Detergents are a class of substances with cleaning properties that have a head and tail region. A figure like this one can be used to represent a generic detergent molecule. In this figure, the yellow circle represents the head region and the wavy blue line represents the tail region. The head region of a detergent molecule may be ionic or nonionic, but it is always classified as hydrophilic, which literally translates to “water loving.” The tail region of a detergent molecule is typically composed of nonpolar hydrocarbon chains and is classified as hydrophobic, which literally translates to “water fearing.”
To answer this question about detergent molecules, we need to determine which illustration best describes the way in which detergent molecules initially accumulate in water. Since the head of a detergent molecule is hydrophilic or water loving, we should expect the head of the detergent molecules to be attracted to the water. And since the tail region of a detergent molecule is hydrophobic or water fearing, we should expect that the tails of the detergent molecules will not be attracted to the water. In fact, the tail region is so hydrophobic that when the detergent is initially added to water, the tail region will be as far from the water as possible.
Looking at the illustrations, we can quickly eliminate illustration (B), as this illustration shows the hydrophobic tails in the water and the hydrophilic heads far from the water, which is the opposite of what we should expect to occur. We can also eliminate illustrations (A) and (D), as both of these illustrations depict both the head and tail of a detergent molecule in the water. As the tail region is water fearing, we should not expect the tail to be attracted to the water. We can also eliminate illustration (C). This illustration shows that the head region of some of the detergent molecules is attracted to the water. But some of the detergent molecules are drawn with the tail region in the water. And as the tail region is water fearing, we shouldn’t expect this to occur.
The illustration in answer choice (E) shows the head region of all of the detergent molecules in the water and the tail region of all of the detergent molecules in the air. As the head region is water loving and the tail region is water fearing, this illustration best describes the way in which detergent molecules initially gather at the surface of the water. So the correct answer is the illustration shown in answer choice (E), where the heads of the detergent molecules are in the water and the tails are in the air.
